You may want to change your modem init strings to better optimize your connection. OH, did you mean maybe a Rockwell modem?
I've used a few USB modems while traveling recently. I have been very happy with the Zoom USB dongle modem. I have seen connect speeds to our dialup network range from 46.6 to 53.
I know your not travelling, but I was surprised at the quality of connection on this USB modem. For internal modems, I still use a 3COM/USR 56K modem from way back.